Ayanamsa

The Earth's axis slowly wobbles, so the point where the Sun crosses the equator in spring moves backward against the stars by about one degree every 72 years. Western astrology measures the zodiac from that moving point (tropical). Indian astrology measures it from the fixed stars (sidereal).

The ayanamsa is the angle between the two. Today it is about 24 degrees, which is why a Sun sign in the sidereal zodiac is usually the sign before the Western one. Different schools fix the starting point slightly differently. This site uses Lahiri (also called Chitrapaksha), the most widely used choice in India and the one adopted for the Indian national calendar.